(1) Field of the Invention
This invention relates to a multistage flash evaporator, and more particularly to such an evaporator which comprises a plurality of horizontally extending cylindrical shells of shell and tube type arrangement provided in parallel spaced relation, tube sheets supported by opposite end portions of the shells, bundles of cooling tubes held between the tube sheets and travelled by cooling water inside, a top plate connecting the upper portions of the shells to one another, internal bulkheads connecting the lower portions of the shells and a bottom plate, and demisters mounted between the shells.
(2) Description of the Prior Art
Conventional multistage flash evaporators are typically as shown in FIGS. 1 to 3. Throughout these figures the vessel is shown built up of plates forming the bottom T, sides S, top V, front W, and rear X. The vessel is partitioned into a number of stages by internal bulk-heads K. The stages thus provided in the multistage flash evaporator include flash chambers A, A.sub.1, A.sub.2, etc. and condenser chambers B, B.sub.1, B.sub.2, etc.
Brine, admitted to the vessel through a brine inlet F, then flows in the direction indicated by the arrow into the flash chamber A, where it partly flashes into vapor, and the vapor passes by routes indicated by the dotted-line arrows into the condenser chamber B. Through a heat exchange with cooling water running in a bundle of cooling tubes C, the vapor is condensed to fresh water or plain water and is taken out via a tray G. The trays G are connected to a distillate trough Y, in known fashion, for the common removal of the distillate from the unit. On the other hand, the balance of brine not vaporized in the chamber A passes onward through interstage brine orifices H into the next-stage flash chamber A.sub.1, where it is handled in the same way as in the first compartment, and thence the unvaporized brine continues to travel from stage to stage. Throughout these figures the symbol D indicates a tube sheet; E, water chambers; I, demisters; J, vent pipes for noncondensable gases; and L, tube support plates.
The multistage flash evaporators of the ordinary designs have the following disadvantages. The internal bulkheads K, which connect the top plate V and the bottom plate T together, have to be tall accordingly and, in order to stand the vertical loads and the forces that result from the pressure difference between the adjacent flash chambers, they should be sufficiently thick-walled, with additional support by a number of reinforcements. In case of the construction as illustrated in FIG. 2, the width of the flash chamber A is often governed by the area the demister I is to occupy, with consequent increases in the overall dimensions of the apparatus to a disadvantage. The construction shown in FIG. 3, by contrast, permits the installation area of the demister to be changed as desired by upward or downward shifting of the demister position. A disadvantage of this arrangement is the division of the demister into two parts, which necessitates an increased number of component parts. Among other disadvantages are that the divided vapor streams toward the left and right demister parts are likely to deviate from normal courses and that the left demister part I is located so close to the brine orifices H that the flashing brine tends to impinge upon that part and affect it unfavorably.